A Platinum Molecular Complex Immobilised on the Surface of Graphene as Active Catalyst in Alkyne Hydrosilylation

Article abstract of DOI:10.1002/ejic.202000356

A platinum complex bearing a N-heterocyclic carbene (NHC) ligand functionalised with a pyrene-tag is immobilised onto the surface of reduced graphene oxide (rGO). The hybrid material composed of an organometallic complex and a graphene derivative is ready

Full text of DOI:10.1002/ejic.202000356

European Journal of Inorganic Chemistry
Accepted Article
Title: A Platinum molecular complex immobilised on the surface of
graphene as active catalyst in alkyne hydrosilylation
Authors: Andres Mollar-Cuni, Pilar Borja, Santiago Martin, Gregorio
Guisado-Barrios, and Jose A Mata
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000356
Link to VoR: https://doi.org/10.1002/ejic.202000356
01/2020

10.1002/ejic.202000356
European Journal of Inorganic Chemistry
FULL PAPER
A Platinum molecular complex immobilised on the surface of
graphene as active catalyst in alkyne hydrosilylation
Andres Mollar-Cuni,[a] Pilar Borja,[a] Santiago Martin,[b] Gregorio Guisado-Barrios,[a] and Jose A. Mata*[a]
[a]
A. Mollar-Cuni, Dr. P. Borja, Dr. G. Guisado-Barrios and Dr. J. A. Mata
Institute of Advanced Materials (INAM), Centro de Innovación en Química Avanzada (ORFEO-CINCA)
Universitat Jaume I
Avda. Sos Baynat s/n, 12006, Castellón, Spain
E-mail: jmata@uji.es
http://www.inam.uji.es/research-divisions/research-division-dr-jose-mata
Dr. S. Martin
[b]
Departamento de Química Física (Facultad de Ciencias), Instituto de Ciencias de Materiales de Aragón (ICMA)
Universidad de Zaragoza-CSIC
C/Pedro Cerbuna 12 50009 Zaragoza, Spain
Supporting information for this article is given via a link at the end of the document.
Abstract: A platinum complex bearing a N-heterocyclic carbene
(NHC) ligand functionalised with a pyrene-tag is immobilised onto the
surface of reduced graphene oxide (rGO). The hybrid material
composed of an organometallic complex and a graphene derivative is
ready available in a single-step process under mild reaction conditions.
This methodology preserves the inherent properties of the active
catalytic centre and the support. The platinum hybrid material is an
efficient catalyst in hydrosilylation of alkynes and can be recycled and
reused without significant loss of activity due to its high stability.
Interestingly, the catalytic activity of the platinum complex is not
affected by diffusion problems after immobilisation. The influence of
graphene in hydrosilylation of alkynes is discussed in terms of activity
and selectivity.
recently been applied for the hydrosilylation of terminal
alkynes.[26–28] On the contrary, the use of non-covalent interaction
for the immobilisation of well-defined complexes onto graphene
allows the preparation of hybrid materials in a single step with
controllable metal positions at specific sites.[29–32] This approach
is complementary to homogeneous systems, since allows the
easy recovery of the catalysts, and at the same time provides
mechanistic information regarding catalysts resting state. In
addition, the immobilisation of metal complexes facilitates the
precise control of the catalytic active sites.[33–35] Based on our
experience on the preparation of different supported catalysts, we
envisioned that the incorporation of a molecular platinum catalyst
onto the surface of graphene could result beneficial for the
hydrosilylation reaction of alkynes by: i) decreasing the kinetics of
the deactivation pathways, ii) hampering the interaction with
external agents and iii) increasing the catalyst activity by
facilitating the interactions between the catalyst and the
substrates.
Introduction
Hydrosilylation of alkynes represents a direct approach for the
synthesis of carbon-silicon bonds.[1–4] These compounds are
useful industrial intermediates for the production of fine chemicals,
silicones, lubricants, coatings and fine chemicals.[5][6] Among all
the metals active in hydrosilylation reactions, platinum stands out
in terms of activity and selectivity. As example, highly active
Karstedt[7] and Speier[8] catalysts, are present in several industrial
processes (Fig. 1). However, they bear limited stability against
oxygen and moisture under catalytic conditions forming platinum
species that can taint the products. In order to circumvent this
problem, Markó et al, introduced a series of N-heterocyclic
carbene ligand (NHC) based platinum complexes (Fig. 1).[9–12]
These homogeneous catalysts have shown remarkable efficiency
and selectivity in the hydrosilylation of alkynes under aerobic
conditions but they cannot be easily isolated and used in multiple
runs.[13–18] Hence, seeking new effective alternatives for catalysts
endurance is highly encouraging. In this context, immobilisation
of molecular metal complexes onto supports enables the
development of hybrid materials with specific and controllable
properties relevant for ulterior catalytic applications.[19–22] Metal
complexes can be anchored onto supports by means of strong
covalent bonds in order to avoid leaching during the catalytic
reactions. This immobilisation process normally requires several
Figure 1. Active platinum catalytic systems for hydrosilylation.
Herein, in this manuscript, we describe a contrastive study of the
structural and catalytic properties of a molecular platinum type
PEPPSI complex 1 (PEPPSI = pyridine-enhanced precatalyst
preparation, stabilisation and initiation) and its related derivative
immobilised onto the surface of reduced graphene oxide (rGO) by
non-covalent interactions 1-rGO (Fig.1). The hybrid catalytic
material is obtained in
a single step starting from an
synthetic steps and lacks of a complete control in the way the
organometallic complex and the carbonaceous material.
[23–25]
metal sites are tethered to supports.
Some of them have
1
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10.1002/ejic.202000356
European Journal of Inorganic Chemistry
FULL PAPER
Scheme 1. Synthesis of the platinum complex 1 and the hybrid material 1-rGO.
The catalytic performance of both, molecular system and the
hybrid material were evaluated towards the hydrosilylation
reaction of alkynes. Metal complex 1 is a competent catalyst
towards terminal and internal alkynes displaying moderate
regioselectivity, having a preferential inclination for  versus the
isomer. In parallel, the catalytic activity of 1 was matched by the
hybrid material 1-rGO, which displayed a greater stability but
present the advantage that can be recycled and reused for up to
ten runs without significant loss of activity.
with two trans bromides, a 3-chloropyridine and the NHC ligand
in square-planar coordination environment (Fig. 2).
Results and Discussion
The platinum molecular complex 1 and the related hybrid material
1-rGO immobilized onto graphene were synthesised as described
in Scheme 1. The aim of this study is the evaluation of the catalytic
properties in hydrosilylation from
a
homogeneous and
heterogeneous approach. Initially, molecular Pt-NHC complex 1
was prepared by applying the same methodology described for
the palladium PEPPSI type complexes.[36–38] Imidazolium salt
functionalised with a pyrene polyaromatic tag[39] used as ligand
precursor was treated with PtCl2, 3-chloropyridine, K2CO3 as
based and excess of NaBr to avoid halide scrambling. Complex 1
was isolated in 70% yield and fully characterised by NMR
spectroscopy, ESI-MS spectrometry, elemental analysis and
thermogravimetric analysis. The complete characterisation at the
molecular level facilitates the monitoring of the changes that the
complex undergoes during the immobilisation process and the
catalytic experiments. Coordination of the NHC ligand to platinum
is confirmed by the disappearance of the acidic signal in 1H NMR
spectra of the imidazolium salt at 9.36 ppm. 13C NMR spectra
showed the characteristic signal for Pt–Ccarbene at 148.3 ppm
although the satellites corresponding to the coupling with 195Pt are
not observed due to their low intensity. Positive ion ESI mass
spectrum analysis (ESI-MS) in acetonitrile of complex 1 shows a
base peak for [M – Br + CH3CN]+ at m/z = 830.3 and a less intense
peak for [M – Br – (3-Cl-Pyridine) + 2CH3CN]+ at m/z = 757.3. The
presence of these fragments confirms the proposed molecular
composition of complex 1 based on the mass/charge relation and
the isotopic pattern. In addition, it reveals the lability of the 3-
chloropyridine ligand, as expected for PEPPSI-type complexes.
Metal–halide bond-breaking is a common ionisation mechanism
for neutral complexes under ESI conditions and L-type ligands
correlate with ease metal-ligand substitution. The molecular
structure of complex 1 was confirmed by single-crystal X-ray
diffraction.[40] The structure analysis reveals a platinum centre
Figure 2. Molecular structure of compound 1. Ellipsoids are at 50% probability
level. Hydrogen atoms and crystallisation solvent (dichloromethane) have been
omitted for clarity. Selected bond lengths [Å] and angles [°]:Pt(1) - C(4) 1.962(3),
Pt(1) - Br(2) 2.4266(3), Pt(1) - Br(3) 2.4173(4), Pt(1) - N(9) 2.097(3), Br(2) - Pt(1)
- Br(3) 176.945(13), N(9) - Pt(1) - C(4) 178.55(12), N(9) - Pt(1) - Br(2) 89.76(8),
C(4) - Pt(1) - Br(2) 89.37(9).
The hybrid organometallic-graphene material 1-rGO was
obtained by applying an immobilisation methodology previously
reported by us.[30,41,42] It is based in the establishment of non-
covalent π-type interactions between the polyaromatic pyrene tag
and the rGO. The tethering of the molecular complex involves
mixing complex 1 with reduced graphene oxide (rGO) in
dichloromethane at room temperature, followed by 30 minutes
sonication in a water bath. Finally, the mixture is stirred at room
temperature for 24 h. The exact platinum content on the hybrid
material 1-rGO was determined by digestion of the samples in hot
mixture of HCl/HNO3 followed by ICP-MS analysis. The results
obtained by ICP-MS analysis accounted for 4.6 wt% of 1 in the
hybrid material 1-rGO.
The characterisation of the platinum hybrid material 1-rGO
was performed using High Resolution Transmission Microscopy
(HRTEM) and X-ray Photoelectron Spectroscopy (XPS). HRTEM
images of 1-rGO at different magnifications (Fig. 3a,b) show the
characteristic 2D morphology of graphene and the absence of
metal nanoparticles. Elemental mapping by Energy-Dispersive X-
ray Spectroscopic analysis (EDS) of 1-rGO (Fig. 3c) confirms the
elemental composition of the molecular complex 1 (Pt, N, Br and
2
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10.1002/ejic.202000356
European Journal of Inorganic Chemistry
FULL PAPER
Catalytic applications
Cl) and the homogeneous distribution of platinum in the hybrid
materials (Fig. 3d). In parallel to this, the surface of hybrid material
1-rGO, was characterised and analysed by means of XPS. This
technique provides relevant information related to the elemental
composition and oxidation states. In our case, a comparative
analysis of complex 1 and hybrid material 1-rGO facilitates the
interpretation of the results. A survey XPS analysis of 1 shows the
presence of platinum, nitrogen, bromide and chloride (Fig. 4). As
expected for the molecular complex 1, the two peaks of Pt4f
confirm the +2 oxidation state at binding energies of 72.7 and 76.0
eV.[43] A high-resolution XPS analysis of 1 and 1-rGO shows the
characteristic core-level peaks of P4 4f, Br 3d, N 1s and Cl 2p at
the same binding energies for the molecular complex and the
hybrid material (Fig. 4). These results confirm the tethering of the
molecular complex 1 onto the surface of graphene. The nature of
the complex is not altered during the harness process.
Additionally, we have evidence of the exact nature and
composition of the platinum species deposited onto the graphene
surface.
The catalytic properties of the molecular platinum complex 1 and
the hybrid material 1-rGO were studied in the hydrosilylation
reaction of internal and terminal alkynes. Optimisation of reaction
conditions was carried out using phenyl acetylene and
dimethylphenylsilane as model substrates (Table 1).
Table 1. Optimisation of reaction conditions for the hydrosilylation of phenyl
acetylene.
Sel.[a]
(/)
Cat.
mol%
Solvent
(1 mL)
T
Time
(h)
Conv
Entry
(oC)
(%)[a]
1
2
-
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
MeCN
50
80
30
50
65
80
90
80
80
80
80
80
80
80
3
3
0
0
-
-
-
3
1
3
0
-
4
1
3
0
-
5
1
3
5
30/70
33/67
32/68
33/67
32/68
35/65
32/68
40/60
41/59
40/60
6
1
3
40
76
29
34
69
⩾99
8
7
1
3
8
1b
0.5
0.5
0.5
0.5
0.5
0.5
3
9
3
10
11
12
13
14
6.5
24
3
Dioxane
Dichloroethane
3
74
10
3
Conditions: 0.3 mmol of alkyne, 0.31 mmol of silane and 0.3 mmol of anisole
used as internal standard. Aerobic conditions. [a] conversion of alkyne and
selectivity determined by GC analyses. [b] KPF6 (3 mol%) was used as additive.
o
Control experiments at two different temperatures 50 and 80 C
respectively were carried out to confirm that the metal complex is
required in the hydrosilylation reaction. No conversion was
observed when the reaction was performed in the absence of the
platinum catalyst 1 (Table 1, entry 1, 2). Then, we evaluated the
activity of the catalyst varying the temperature (Table 1, entries 3
- 7). Using 1 mol% of catalyst, the reaction does not occur below
50 ºC and it is very slow at 65 ºC (3h, 5% yield). Increasing the
temperature to 80 – 90 ºC affords moderate to good conversions
after 3h. According to these results, the lowest temperature to
study the hydrosilylation reaction under reasonable experimental
conditions was 80 oC. The addition of a halide abstractor (KPF6)
did not improve the catalytic outcomes (Table 1, entry 8).
Reduction of catalyst loading to 0.5 mol% allows to monitor the
reaction into a reasonable time-scale of 24h (Table 1, entries 9 -
11). The influence of catalyst loading in hydrosilylation is
described using reaction progress profiles in the supporting
information (Figure S16). We evaluated the solvent influence and
observed that hydrosilylation also took place with solvents other
than toluene such as MeCN, dioxane and dichloroethane (Table
1, entries 12 - 14). It is worth mentioning that although higher
activity was observed when using dioxane, lower selectivity was
obtained (Entry 13). The influence of the presence of air or
moisture was assessed. No significant catalytic difference in the
presence of air was observed, suggesting a relative high stability
of catalysts precursor and intermediates versus oxygen. The
optimal results in terms of activity and selectivity for the study of
Figure 3. HRTEM images of 1-rGO at different magnifications (a,b). STEM
images (c) and EDS elemental mapping (d) images showing the homogeneous
distribution of platinum on the hybrid material 1-rGO.
Figure 4. XPS analysis of the core-level peaks Pt4f, Br3d N1s and Cl2p for
complex 1 (top) and hybrid material 1-rGO (bottom).
3
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10.1002/ejic.202000356
European Journal of Inorganic Chemistry
FULL PAPER
hydrosilylation reaction were obtained using a catalyst loading of
0.5 mol%, toluene as solvent at 80 oC under aerobic conditions.
Next, we studied the scope and limitations in hydrosilylation of
different terminal and internal alkynes under these reaction
conditions.
the hydrosilylation of the small ethyl phenyl acetylene gave
moderate selectivities (Table 3, entries 1 – 2). In contrast, when
using a more steric demanding trimethylsilane phenyl acetylene
the regioselectivity was 100% and only the  isomer was
observed (Table 3, entries 5 – 7). These results suggest that steric
factors of the internal alkyne dominate the selectivity of the
reaction.
Table 2. Hydrosilylation of terminal alkynes.
Table 3. Hydrosilylation of internal alkynes.
Sel. (%)[a]

Yields
(%)[a]
Entry
R
R’3SiH
Cat.
1
1
2
Ph
Ph
Me2PhSiH
Me2PhSiH
Et3SiH
91
96
35/65/0
43/57/0
65/35/0
30/70/0
65/35/0
40/60/0
32/68/0
42/58/0
30/70/0
25/75/0
38/62/0
37/63/0
39/61/0
35/65/0
Sel.
Yield
Entry
R
R’
R’’3SiH
Cat.
t(h)
(%)[a]
(%)[a]
1-rGO

1
3
Ph
94
1
Me2PhSiH
Et3SiH
1
2
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
nPr
nPr
nPr
Et
Et
2.5
4
99
82/18
p-tolyl
p-tolyl
Benzyl
Benzyl
Benzyl
n-Hex
n-Hex
n-Hex
(CH2)3CN
(CH2)3CN
(CH2)3CN
1
4
Me2PhSiH
Et3SiH
80
1
⩾99
93
81/19
84/16
79/21
100/0
100/0
100/0
1
5
92
1
Me2PhSiH
Et3SiH
3
nPr
2
1
6
Me2PhSiH
Me2PhSiH
Et3SiH
⩾99
⩾99
93
1
4
nPr
3
⩾99
86
1-rGO
7
1
(CH3)3Si
(CH3)3Si
(CH3)3Si
Ph
Me2PhSiH
Me2PhSiH
Et3SiH
5
2.5
2.5
6
1
8
1-rGO
6
87
1
9
Me2PhSiH
Me2PhSiH
Et3SiH
94
1
7
96
1-rGO
10
11
12
13
14
94
b
1
Me2PhSiH
Et3SiH
8
1
⩾99
92
-
1
⩾99
82
b
1
9
Ph
6
-
1
1-rGO
1
Me2PhSiH
Me2PhSiH
Et3SiH
b
1
Me2PhSiH
Me2PhSiH
Et3SiH
10
11
12
13
14
15
nPr
1.5
1.5
3.5
4
⩾99
⩾99
95
-
90
b
1-rGO
nPr
-
87
b
1
nPr
-
b
1
1-rGO
1
MeO(CH2)2 MeO(CH2)2
MeO(CH2)2 MeO(CH2)2
MeO(CH2)2 MeO(CH2)2
Me2PhSiH
Me2PhSiH
Et3SiH
92
-
Reaction conditions: 0.3 mmol of alkyne, 0.31 mmol of silane, 0.5 mol% of
catalyst (based on Pt), T = 80 ⁰C, 20h and 1 mL of toluene. Aerobic conditions.
[a] Yields were determined with GC using anisole as a standard and selectivity
by1H NMR.
b
3
95
-
b
6
98
-
Regioselectivity () vs. the Ph group except for symmetric alkynes. E/Z
isomerism not determined. Reaction conditions: 0.3 mmol of alkyne, 0.31 mmol
of silane, 0.5 mol% of catalyst, T = 80 ⁰C and 1 mL of toluene. Aerobic conditions.
[a] Yields were determined with GC using anisole as a standard and selectivity
by 1H NMR. [b] Only one regioisomer is possible.
We first explored the hydrosilylation of aryl (Table 2, entry 1-5)
and alkyl terminal alkynes (Table 2, entry 6-14) using complex 1
and the hybrid material 1-rGO. Both catalytic systems are
competent in the hydrosilylation of alkynes affording quantitative
yields after 20 h reaction. Preliminary comparison experiments of
1 vs. 1-rGO for different substrates reveal that the catalytic activity
is maintain or slightly improved (Vide infra). We have observed
that substrate scope is maintained after immobilization (Table 2,
entries 2,7,10,13). Improvement of regioselectivity is a difficult
task in hydrosilylation. In our case, we have observed that the
average regioselectivy is 40/60 for the -addition products
(Table 2). We have observed that regioslectivity is maintained
after catalysts immobilization. The best results were obtained in
the case of 1-hexyne displaying a regioselectivity ratio of 25/75
for the -addition products using catalysts 1-rGO (Table 2, entry
10). Interestingly a very high stereoselectivity towards the (E)
versus the (Z)-addition product was observed. In fact, no
formation of (Z)-isomer was detected in any case.
Then, we studied the performance of complex 1 and the
hybrid material 1-rGO in hydrosilylation of a variety of internal
alkynes using different silanes (Table 3), resulting an efficient
catalysts. Yields over 90% were obtained for internal alkynes in
ca. 3 h, much faster than the previously observed for the terminal
alkynes. The regioselectivity for non-symmetric internal alkynes
favors the addition of the silyl group at the C atom bound next to
the phenyl group (defined as the -addition product). We have
observed that the regioselectivity is controlled by sterics imposed
by the alkyne rather than the nature of the silane. For instance,
Next, different competition experiments were carried out to
explore the selectivity of alkyne hydrosilylation in the presence of
other substrates bearing different functional groups such as
alkenes, aldehydes, and nitriles (Fig. S13 – S15). It was found
that the platinum catalyst 1 produced the hydrosilylation of
alkynes without altering any of the other substrates bearing
aldehydes, alkenes or nitriles. It is important to note that the
kinetics of hydrosilylation are strongly affected by the presence of
other functionalities and in particular the induction time is
completely different. The results suggest that complex 1 is a
competent catalyst in the hydrosilylation of alkynes also in the
presence of other functional groups.
We assessed the effect of catalyst loading in the
hydrosilylation of 1-phenyl-1-butyne with dimethylphenylsilane
using 1 (Fig. S16). The reaction progress profiles showed that
quantitative yields are obtained for catalyst loadings between 2 –
0.01 mol%. These experiments reveal that the induction time is
dependent of the catalyst loading. The maximum catalyst turnover
number (TON) of 9.3 x 103 was obtained at 0.01 mol% loading
after 24h. The best catalyst turnover frequency (TOF) at 50%
conversion reached a remarkable 380 h-1 and was achieved when
using a catalyst loading of 0.01 mol%. These results reveal that 1
is a robust and stable catalyst. In order to analyse the nature of
the catalytic active species we performed poisoning experiments
(ESI). We used poly(4-vinylpyridine) (PVP)[44] as an scavenger of
4
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10.1002/ejic.202000356
European Journal of Inorganic Chemistry
FULL PAPER
Pt(II) molecular species and Hg (Mercury test)[45] as an scavenger
of Pt nanoparticles.[46] The addition of Hg increases the time of the
induction period from 50 to ca. 100 min (Compare Fig. S12 vs
S17). The results reveal that the addition of mercury had no effect
in hydrosilylation but the addition of PVP completely inhibits the
reaction (Fig. S17). The poisoning experiments suggest that the
catalytic active species are homogeneous in nature.
model reaction. After 2 h reaction (GC conversion 54%), half of
the reaction mixture was separated by filtration at 80 ⁰C. The GC
analysis after 15 h indicates a slight increment for the filtrate vs.
quantitative conversion in the case of the supported catalysts 1-
rGO. Preliminary results in the hot filtration test support the
absence of catalytic species in solution due to the release of the
molecular complex from the surface of graphene. These results
suggest that the catalytic process is heterogeneous in nature and
take place at the surface of graphene.
Once we studied the reaction scope and limitations of
complex 1 and the hybrid material 1-rGO, we explored the
catalytic properties of both systems by means of reaction
progress profiles in the hydrosilylation of phenyl acetylene (Fig.
5). These experiments provide valuable information of apparent
reaction rates, induction time and influence of the support in the
catalytic reaction. Both reactions were carried out under the exact
conditions and using the same catalyst loading (0.5 mol%). The
reaction progress profiles reveal that the supported catalyst is
more active that the molecular version in terms of reaction rates
(TOF) and product formation (TON). For instance, the yield is ca.
80% for the hybrid material 1-rGO and 56% for the molecular
complex 1 after 5h. Importantly, immobilization of 1 onto graphene
aided to reduce the induction period significantly as show in the
reaction profile (Fig. 5). These results indicate that the activity of
catalyst 1 is not affected by diffusion problems when supported
onto graphene. In general, diffusion problems are especially
important when using supported catalysts onto porous materials.
The 2D structure of graphene avoids or at least limits the
problems related to diffusion issues. We have observed that this
process is general and it has been experienced for other catalytic
systems using graphene as support.[47–53] The catalytic activity of
the molecular platinum complex improves when supported onto
graphene.
Recycling studies
In view of the good results of complex 1 in the hydrosilylation of
alkynes, we decided to test the recyclability properties of the
hybrid material 1-rGO in order to determine the influence of the
support.[57] Recycling experiments were carried out using
phenylacetylene and dimethylphenylsilane as the model
substrates. The reactions were performed using the conditions
described in the general procedure (Table 1), the reaction
1
progress was monitored by GC and the selectivity by H NMR.
After each run, the mixture was cooled down to room temperature.
The solid catalyst was isolated by decantation, washed
thoroughly with toluene (3 x 5 mL) and used again in another run.
The reaction progress profiles allow comparing the apparent
reaction rates is a direct method to observe catalyst deactivation.
The hydrosilylation of phenylcetylene with dimethylphenylsilane
was monitored for ten runs. The reaction progress profiles reveal
that the hybrid material 1-rGO was reused up to ten times (Fig.
6a). The reaction progress profiles overlapped for seven runs
within the experimental error (<10%). Run 6 should be considered
as an outlier because the catalyst activity is even lower than for
runs 7, 8 and 9 (Figure 6a, green line with triangles). This artefact
is most probably due to an experimental error in the GC
measurement. In order to evaluate the mechanism of the
deactivation process, the platinum content in the solution of each
run was analysed by ICP-MS. The results reveal that the platinum
content in the solutions corresponding to runs 1 – 10 is very low.
These results suggest that deactivation of the hybrid material 1-
rGO is produced by catalyst manipulation during the recycling
experiments. It is important to note that we have not observed a
sharp catalyst deactivation. In addition, the selectivity of
hydrosilylation is not affected in the recycling experiments (Fig.
6b). The : addition product ratio is maintained ca. 40:60 (at 5h
reaction) during all runs. Interestingly, the -(Z) product is not
observed in any run. These results suggest that the nature of the
catalytic active species is preserved from batch to batch. We
believe, that these results are excellent in terms of recycling
especially if we consider the catalyst loading used of 0.5 mol%.
Figure 5. Reaction progress profile in the hydrosilylation of phenylacetylene
using catalysts 1 and 1-rGO. Reaction conditions: 0.3 mmol of phenylacetylene,
0.31 mmol of PhMe2SiH, 0.5 mol% of catalyst (based on Pt), T = 80 ⁰C and 1
mL of toluene. Conversion by GC analysis using anisole as a standard.
A common problem generally encountered in supported catalysis
is the release of the catalytic active species into the solution
during the reaction, known as the “boomerang effect”.[54,55] In
order to evaluate the mechanism of action in the immobilisation of
molecular species onto graphene we performed hot filtration
experiments.[56] We used catalyst 1-rGO (0.5 mol%) in the
hydrosilylation of phenylacetylene and dimethylphenylsilane as a
5
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10.1002/ejic.202000356
European Journal of Inorganic Chemistry
FULL PAPER
Figure 7. HRTEM images of 1-rGO at different magnifications (a,b) after ten
catalytic runs. STEM image showing the absence of platinum nanoparticles (c)
and EDS elemental mapping (d) image shows the homogeneous distribution of
platinum on the hybrid material 1-rGO after ten catalytic runs.
Figure 6. Recycling properties of 1-rGO in hydrosilylation. a) Activity using
reaction progress profiles for runs 1 to 10. b) Selectivity in the hydrosilylation of
phenylacetylene at 5h reaction. Conversions determined by GC analysis and
selectivity by 1H NMR.
XPS analysis comparison of the hybrid material 1-rGO before and
after ten catalytic runs (Fig. 8) displayed the expected peaks
corresponding at Pt4f, Br3d and N1s at the same binding energies.
The high-resolution XPS analysis for the core-level peak of Pt4f,
which appears as a doublet, indicates the presence of platinum
only in the +2 oxidation state. These results demonstrate that after
ten catalytic runs the platinum species observed are similar to the
initial ones. XPS analysis also suggests that the nature and
composition of catalyst resting state should be similar to the
complex 1. Thus, the hybrid material 1-rGO is a robust solid
catalyst that can be reused maintaining the selectivity in
hydrosilylation.
We analysed the hybrid catalytic material 1-rGO after the
recycling experiments in order to explore the changes of the
support and complex 1 and get important information about the
reaction mechanism and/or deactivation pathways. After ten
catalytic runs, HRTEM images of the hybrid material 1-rGO
showed that the morphology of the graphene is maintained after
the reactions. The only difference that we noticed was the
presence of more wrinkles as consequence of the usage of the
hybrid material in the catalytic experiments (Fig. 7). The dark field
STEM images confirmed the absence of platinum nanoparticles
and the EDS elemental mapping showed the homogeneous
distribution of platinum on the hybrid material 1-rGO after ten
catalytic runs as it was in the initial material. These results agree
with the poisoning experiments indicating that platinum
nanoparticles are not involve in the hydrosilylation reaction.
Figure 8. XPS analysis comparison of the hybrid material 1-rGO before (top)
and after ten catalytic runs (bottom).
6
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10.1002/ejic.202000356
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis of 1. Platinum chloride (200 mg, 0,75 mmol), imidazolium salt
functionalised with a pyrene-tag (361,3 mg, 0,75 mmol), potassium
carbonate (521,3 mg, 3,75 mmol) and sodium bromide (780,0 mg, 7,5
mmol) were added to a Schlenk flask under nitrogen atmosphere. Then,
2,2 mL of 3-chloropyridine were added, and the mixture was stirred at 80
Conclusions
A direct method for the immobilisation of metal complexes onto
the surface of graphene by non-covalent interactions allows the
precise control of the catalytic properties of the corresponding
hybrid material. The structure of the metal complex and the
properties of the support are not altered during the immobilisation
process due to mild conditions used. The hybrid material contain
metal sites homogeneously distributed and not only located at
edges of defects. The catalytic studies with the molecular
platinum complex 1 and the hybrid material 1-rGO in the
hydrosilylation reaction of alkynes reveal some of the advantages
of the immobilisation. The catalytic activity and stability of the
molecular complex 1 is maintained when anchored onto graphene
while greater reaction rates were observed. Most probably the
support prevents some deactivation pathways and aid to bring in
close proximity substrates with the metal sites at the surface of
graphene facilitating their interaction. Nevertheless, the platinum
hybrid material (1-rGO) is an efficient catalyst in hydrosilylation of
alkynes allowing recycling and reuse. This work represents a
clear example of the catalytic benefits observed in the
immobilisation of metal complexes onto graphene. The catalytic
improvement in terms of activity and stability may inspire future
research in development of efficient supported catalysis.
⁰
C for 43 h. After solvent removal, the crude product was purified by column
chromatography. The pure compound 1 was eluted using CH2Cl2 and was
precipitated as a yellow solid from a mixture of CH2Cl2/hexane in 69 %
4
yield. 1H NMR (400 MHz, CDCl3): δ 8,96 (d, JH-H = 2,2Hz, 1H, CH Cl-Py),
8,84 (d,3JH-H = 5,6Hz, 1H, CH Cl-Py), 8,67 (d, 3JH-H= 9,3Hz, 1H, CH pyr), 8,21
3
(m, 5H, CH pyr), 8,10 (m, 2H, CH pyr), 8,03 (t, JH-H = 6,3Hz, 1H, CH pyr),
7,65 (ddd, 3J1 H-H = 8,3Hz 4J2 H-H= 2,2Hz 4J3 H-H = 1,3Hz, 1H, CH Cl-pyr), 7,18
3
(m, 1H,CH Cl-Pyr), 7,03 (s, 2H, CH mes), 6,69 (s, 2H, N-CH2), 6,62 (d, JH-H
= 2,0Hz, H, CH imid), 6,53 (d, 3JH-H = 2,0Hz, 1H, CH imid), 2,38 ( s, 3H, CH3
mes), 2,34 ( s, 6H, CH3 mes). 13C NMR (400 MHz, CDCl3): δ 151,7 (CH Cl-
Py ), 150,7 (CH Cl-Py ), 148,9 (C pyr), 137,5 (CH Cl-Py), 137,4 (Ccarbene- Pt),
136,1-123,6(CH Cl-Py, , C Cl-Py, CH pyr, C pyr, CH mes, C mes), 123,3 (CH imid),
120,2 (CH imid), 53,8 (N-CH2), 21,2 (CH3 mes), 19,5 (CH3 mes).Electrospray
MS (Cone 20 V) (m/z, fragment): 788.0 [M-Br]+.
Synthesis of the hybrid material 1-rGO. In a round-bottom flask were
introduced 180 mg of rGO and 100 mL of CH2Cl2. The suspension was
introduced in ultra-sounds during 30 minutes. Then, 25 mg of compound
1 was added to the exfoliated graphene, and the mixture was sonicated
for 5 extra min. Next, the suspension was stirred at room temperature for
12 hours. The mixture was filtered and washed with CH2Cl2 (3 x 25 mL)
obtaining a black solid (material 1-rGO). The filtrates were combined and
evaporated to dryness under reduced pressure. The presence of
unsupported complex 1 was analysed by 1H NMR using anisole as internal
standard. Integration of the characteristic signal of anisole accounts for a
first indication of the complex amount deposited onto rGO. The exact
amount of complex supported was determined by ICP-MS analysis.
Digestion of the materials was performed in a microwave oven using nitric
and hydrochloric acids (3:1). The exact amount of complex supported was
determined by ICP-MS analysis. This analysis shows that the quantity of
complex 1 in the material 1-rGO is 4.55% in weight.
Experimental Section
General procedures. Anhydrous solvents were dried using a solvent
purification system. The imidazolium salt functionalised was prepared
according to reported procedures.[39] Nuclear magnetic resonance (NMR)
spectra were recorded on Bruker spectrometers operating at 300 or 400
MHz (1H NMR) and 75 or 100 MHz (13C{1H} NMR), respectively, and
referenced to SiMe4 ( in ppm and J in Hertz). NMR spectra were recorded
at room temperature with the appropriate deuterated solvent. Elemental
analysis were carried out in a TruSpec Micro Series. Electrospray Mass
Spectra (ESI-MS) were recorded on a MicroMass Quatro LC instrument.
MeOH was used as mobile phase and nitrogen was used as the drying
and nebulizing gas. High-resolution images of transmission electron
microscopy (HRTEM) and high-angle annular dark-field (HAADF-STEM)
images of the samples were obtained using a Jem-2100 LaB6 (JEOL)
transmission electron microscope coupled with an INCA Energy TEM 200
(Oxford) energy dispersive X-Ray spectrometer (EDX) operating at 200 kV.
Samples were prepared by drying a droplet of a MeOH dispersion on a
carbon-coated copper grid. X-ray photoelectron spectra (XPS) were
acquired on a Kratos AXIS ultra DLD spectrometer with a monochromatic
Al Kα X-ray source (1486.6 eV) using a pass energy of 20 eV. To provide
a precise energy calibration, the XPS binding energies were referenced to
the C1s peak at 284.6 eV. GC analyses were obtained on a Shimadzu GC-
2010 apparatus equipped with a FID detector, and using a Teknokroma
column (TRB-5MS, 30 m x 0.25 mm x 0,25 m). UV-vis spectra were
acquired on a Varian Cary 50 spectrophotometer.
Acknowledgements
The authors thank the financial support from MICIU (RTI2018-
098237-B-C22 and RTI2018-098903-J-I00), and Universitat
Jaume I (UJI-B2018-23). A. M. thanks the Generalitat Valenciana
(AICO/2015/039) for a grant. S.M. acknowledges DGA/fondos
FEDER (construyendo Europa desde Aragon) for funding the
́
research group Platon (E31_17R). The authors are very grateful
́
to the ‘Servei Central d’Instrumentació Científica (SCIC)’ of the
Universitat Jaume I as well as to Dr. G. Antorrena for technical
support in the XPS studies.
Notes
CCDC1966283 contains the supplementary crystallographic data
for this paper. These data are provided free of charge by The
Cambridge Crystallographic Data Centre.
Catalytic hydrosilylation experiments. A pyrex© tube with a stirring bar
is charged with 0.3 mmol of alkyne, 0.31 mmol of hydrosilane, catalyst and
1 mL of toluene as solvent. The tube is then introduced in a preheated 80
oC oil bath. Yields and conversions were determined by GC and/or 1H NMR
analysis using anisole or 1,3,5-trimethoxybenzene as internal standard.
Recycling experiments were carried out under the same reaction conditions as
described in the general procedure. After completion of each run (24 h), the
reaction mixture was allowed to reach room temperature and the catalyst
was isolated by decantation. The solid was washed thoroughly with
toluene (3 x 5 mL) and toluene (3 x 5 mL) and reused in the following run.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Supported catalysis • hydrosilylation • graphene •
Platinum • N-heterocyclic carbenes (NHC)
7
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10.1002/ejic.202000356
European Journal of Inorganic Chemistry
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Entry for the Table of Contents
Supported Catalysis
Development of a hybrid material composed by a well-defined platinum complex and graphene is described. The immobilization is
achieved in a single-step by functionalization with a pyrene-tag and thanks to the formation of π-staking interactions. This
methodology paves the way for a direct access of solid catalysts that allow catalyst recycling and improve catalyst efficiency.
9
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